Thermionic dispenser cathode



Dec. 9, 1958 P. P. COPPOLA THERMIONIC DISPENSER CATHODE Filed Aug. 15,1955 INVENTOR. H4 TRICK P C'OPPOLA AGE United States Patent THERMIONICDISPENSER CATHODE Patrick P. Coppola Dobbs Ferry N. Y. assi nor to NorthAmerican Philips Compahy, Inc, Nevi' York, N. Y., a corporation ofDelaware Application August 15, 1955, Serial No. 528,296

3 Claims. 01. 31 3-346) My invention relates to a thermionic dispensercathode. In particular my invention relates to an improvement n athermionic dispenser cathode of the type disclosed in U. S. Patent No.2,700,118 to R. C. Hughes et al. As described in that patent, thecathode comprises a pressed and sintered mixture of a refractory metalsuch as tungsten, molybdenum, tantalum, hafnium or alloys of thosemetals and an alkaline earth material which reacts with the refractorymetal upon heating to furnish alkaline earth metal to an emissivesurface of the cathode. As alkaline earth materials, fused mixtures ofan alkalineearth oxide and an acid forming oxide are disclosed whichreact with the refractory metal to furnish alkaline earth metal, e. g.barium, to the emissive surface of the cathode.

Since the alkaline earth material is distributed throughout a porousbody of refractory metal during operation of the cathode some alkalineearth metal or oxides, or both, are volatilized because of their lowvapor pressure in vacuum. The evaporation of alkaline earth metal oroxides, or both, is of course, generally undesirable because thoseproducts may be deposited on other electrodes of an electron-dischargetube where they give rise to secondary emission.

Accordingly, it is a principal object of my invention to provide adispenser cathode of the aforesaid type having a reduced rate ofevaporation of alkaline earth metal and/0r oxides.

A further object of my invention is to provide a dispenser cathodehaving a high emissivity and a low rate of evaporation of alkaline earthmetal oxides, or both.

These and further objects of my invention will be apparent as thespecification progresses.

In accordance with the invention, I have found, quite unexpectedly, thatif portions of the emissive surface are covered with a non-porousrefractory metal, the rate at which alkaline earth metal is supplied tothe emissive surface is still sufiicient to maintain coverage of thatsurface with alkaline earth metal so that high emission can bemaintained. However, since a portion of the pores in that surface arenow covered, the rate of evaporation of the alkaline earth metal oroxides, or both, is materially reduced.

Thus, in accordance with my invention, I cover the emissive surface ofthe cathode with a non-porous member of refractory metal such asmolybdenum, having a number of apertures suitably spaced to permit thealkaline earth metal to migrate over and substantially cover the surfaceof the cathode. Thus, I may use a plurality of interlaced spaced stripsor wires of molybdenum forming a mesh-like structure. Alternatively, Imay use a template having a number of apertures suitably spaced topermit alkaline earth metal to migrate onto the emissive surface whilealso reducing the number of pores in the emissive surface.

The spacing between apertures in the non-porous covering memberpreferably should not exceed the migration distance of the alkalineearth metal over the refractory metal surface in order that maximumsurface cov- 2,864,028 Patented Dec. 9,1958

erage on the emissive surface may, at all times, be maintained. Thismigration distance will depend upon :a .number of factors including theparticular alkaline earth metal being supplied to the surface, theoperating temperature of the cathode, and the refractory metal. Forexample,

, at 1100 C. brightness, the migration length of Ba or BaO, or both, onmolybdenum is about 0.3 mmpwhile at 900 C brightness the distanceincreases to 0.6 mm.

In the preferred embodiment of my invention a source of barium isprovided in the pores of the cathode and molybdenum strips or wires areembedded in the emissive surface. These strips or wires are spacedsufficiently far apart, e. g. at least 0.5 mm., to assure substantiallyfull surface coverage of barium during operation of the cathode.

The invention will be described in greater detail with reference to theaccompanying drawing in which:

Fig. 1 is an elevational view in section of a cathode according to theinvention;

Fig. 2 is a top view of the cathode showing the emissive surface; and

Fig. 3 is a top view of another embodiment of another cathode showingthe emissive surface.

The cathode shown in Fig. 1 comprises an emissive wafer I mounted at oneend of a molybdenum sleeve 2 which houses a heater 3. Embedded in theemissive surface of the wafer are strips 4 which form a grid-likestructure shown in Fig. 2.

The emissive wafer comprises a pressed and sintered body of about 90% byweight of a 25% tungsten to molybdenum alloy and the balance,distributed throughout the pores of the body, a fused mixture of about 5moles of barium oxide and 2 moles of aluminum oxide. Prior to thepressing operation, strips or wires of molybdenum having a diameter ofabout .3 mm. are placed on top of the powder about .5 mm. apart. Themixture is pressed and sintered at 1370 C. brightness for about 20minutes and then rapidly raised to the melting point of the fused oxides(about 1650 C. to 1750 C.). The molybdenum strips are, therefore,integrally united with the sintered body and provide zones in which thesurface pores are completely closed. Thus, those zones contribute nobarium metal to the surface but barium metal which is supplied to thesurface by the pores in the exposed areas migrates over the molybdenumstrips and thereby contributes to the emission from the surface.

As shown in Fig. 3, I may use a molybdenum plate 5, having a number ofsuitably spaced apertures 6 covering a substantial portion of theemissive wafer 1. The molybdenum plate may be sintered to the emissivewater 1 or may be welded to the molybdenum sleeve 2 at the tips of thecusps 7. The covered portions of the wafer are supplied with bariumthrough the apertures in the plate. The apertures are suitably spaced sothat substantially full surface coverage can be maintained at all timesby the migration of barium over the surface.

While I have described my invention in connection with specific examplesand applications, other modifications thereof will be apparent to thoseskilled in the art without departing from the spirit and scope of theinvention as defined in the appended claims.

Wherever the word wires appears in the appended claims, I wish it to beclearly understood that the term includes not only circular strands butalso flat ribbons, bars and strips having lengths substantially greaterthan either the width or thickness of the element.

What I claim is:

1. A dispenser cathode comprising a porous sintered tungsten body, asupply of a fused mixture of barium oxide and aluminum oxide containinga molar excess of barium oxide within the pores of said body, and aplurality of spaced apart molybdenum strips embedded in a 3 surface ofthe body constituting the emissive surface of that body, said stripsbeing integrally united with the sintered body to provide zones in whichsurface pores are completely closed, said strips having a width notsubstantially exceeding the migration distance of barium overmolybdenum.

2. A dispenser cathode comprising a porous sintered body of an alloy ofmolybdenum and tungsten, a supply of a fused mixture of barium oxide andaluminum oxide containing a molar excess of barium oxide within thepores of said body, and a plurality of spaced apart molybdenum stripsembedded in a surface of that body constituting the emissive surface ofthe body, said strips being integrally united with the sintered body toprovide zones in which surface pores are completely closed, said stripshaving a width not substantially exceeding the migration distance ofbarium over molybdenum.

3. A dispenser cathode comprising a poroussintered body of an alloy ofabout 75% of molybdenum and about 25% tungsten, a supply of a fusedmixture of about 5 moles of barium oxide and 2 moles of aluminum oxideWithin the pores of said body, a plurality of spaced apart molybdenumstrips embedded in a surface of the body constituting the emissivesurface of the body, said strips being integrally united with thesintered body to provide zones in which surface pores are completelyclosed, said strips having a width not substantially exceeding themigration distance of barium over molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS

1. A DISPENSER CATHODE COMPRISING A POROUS SINTERED TUNGSTEN BODY, ASUPPLY OF A FUSED MIXTURE OF BARIUM OXIDE AND ALUMINUM OXIDE CONTAININGA MOLAR EXCESS OF BARIUM OXIDE WITHIN THE PORES OF SAID BODY, AND APLURALITY OF SPACED APART MOLYBEDENUM STRIPS EMBEDDED IN A SURFACE OFTHE BODY CONSTITUTING THE EMISSIVE SURFACE OF THAT BODY, SAID STRIPSBEING INTEGRALLY UNITED WITH THE SINTERED BODY TO PROVIDE ZONES IN WHICHSURFACE PORES ARE COMPLETELY CLOSED, SAID STRIPS HAVING A WIDTH NOTSUBSTANTIALLY EXCEEDING THE MIGRATION DISTANCE OF BARIUM OVERMOLYBDENUM.